1. Field of the Disclosure
This invention pertains to a method for rebalancing a solvent solution useful for treating a photosensitive printing element.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing plates are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. The photosensitive element usually includes a release layer to facilitate release of the coversheet and/or a mask film. The photosensitive elements are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Typically, the element is imagewise exposed to actinic radiation, such as ultraviolet (UV) radiation, through a mask to form exposed areas of the photopolymerizable layer that crosslink or cure and unexposed areas, i.e., unpolymerized material, of the photopolymerizable layer that do not crosslink or harden. Since the unexposed areas remain soluble to solvents and the exposed areas become insoluble to solvents, the imagewise exposed element is washed or developed in a solvent which can remove the unpolymerized area while leaving the polymerized (cured) material intact. The solvent solution may be applied to the photosensitive element in any convenient manner such as by pouring, immersing, spraying, or roller application. Brushing can facilitate the solvent washout process and aid in the removal of the unpolymerized or uncrosslinked portions of the composition.
Preparation of Relief Printing Forms by Development of the photopolymerizable element with washout solutions is well known. Solvent developers typically used in such processes include chlorohydrocarbons, such as trichloroethylene or tetrachloroethylene; aromatic hydrocarbons, such as benzene or toluene; saturated cyclic and acyclic hydrocarbons, such as hexane and cyclohexane; unsaturated cyclic hydrocarbons, such as terpenoid compounds, and lower aliphatic ketones. The solvents may be used alone, in combination, or in a mixture with a non-solvent, i.e., a material that does not dissolve the unpolymerized material. In some cases, the washout or development solution is a mixture of a solvent (to the unpolymerized material) and the non-solvent is an alcohol which aids in the removal of the material forming the release layer, and/or suppress swelling of the photosensitive resin composition cured by exposure to actinic radiation.
A recent trend in development solutions is the composition to have more than two components. Such multi-component development solutions can include additional components to reduce the cost of the development solution, and/or provide additional features or functionality to the development solution, and/or provide features for the photosensitive element. For example, diluents can be used to reduce the cost of the development solution and/or lower odor. Surfactants can be used to stabilize development solution mixture, and/or minimize removed materials from adhering back to the printing form. Fluoropolymers and/or silicone compounds can be used to aid in the ink release and/or clean printing of the resulting printing form.
With use, the development solution becomes contaminated with unpolymerized material and other materials that release from the photosensitive element during development or washout. To reduce costs and environmental impact, it is desirable to recycle the development solution for subsequent development processes. In many cases, contaminated development solution is distilled to separate the contaminants, i.e., unpolymerized material and other materials, from the reclaimant solution of the solvent and optional non-solvent. In some cases, contaminated development solution is mechanically separated by centrifuge and optionally filtration to separate the contaminants from the reclaimant solution.
However, when the development solution is composed of two or more components a problem arises with the use of the reclaimant solution in that the proportion of the components in the reclaimant solution is oftentimes different than the proportion of the components in the starting or fresh development solution, that is, the reclaimant solution is out-of-balance. Loss of the components can be the result from the particular process used to recover the development solution. Also, the separated contaminates, i.e., unpolymerized material, can retain some of the solvent component or other components from the development solution. Although the reclaimant solution having different proportions of components (than the proportions of components in a fresh development solution) may still be capable of removing material from photosensitive elements, its effectiveness at removing materials from photosensitive elements would diminish over time and with continued reuse of the reclaimant solution. Washout with a development solution that is out of balance can result in incomplete removal of unpolymerized material from the photosensitive element such that relief areas in the photosensitive element are not fully formed or incomplete removal of one or more other layers, such as a release layer, of the photosensitive element. Time in the development solution may increase to compensate for incomplete removal. But increased development time generally also increases the time that the photosensitive element undergoes brushing, which can lead to mechanical damage to the relief structure of the resulting print form. Increase development time can also increase the swelling of the photosensitive element by the solvent, and hence longer drying times as well. Usually performance of the development system is optimum when the proportions of the components in the development solution are maintained relatively constant rather than fluctuate within a possible operating window.
In order to maintain the proportions of the components in the development solution and its effectiveness at removing materials from the photosensitive element, it is a common practice to “buckup” the reclaimant solution by adding typically one solution having one or more of the components (of the development solution) to the reclaimant solution that is recycled to the processor. In some cases fresh solvent solution that has all components of the starting solvent solution is added to the cleaned solvent solution to replenish the washout solution in the processor. The buckup solution may contain only the solvent component of the development solution or, a mixture of the solvent component and other component/s in proportions that reflects average use of the development solution. But the development solution can be used to process different sizes and types of photosensitive elements each having different photopolymerizable compositions each with different affinity for the solvent and other components in the development solution. As a result, each customer may have a distinct reclaimant solution (and mix of development solution and reclaimant) based at least on the mix of photosensitive elements processed, the process used to recover the reclaimant, and the developer solution composition. As such, a buck-up solution having proportions of components based upon an average processing system may not sufficiently correct the reclaimant solution to an effective development solution.
In some cases a user measures one property, such as specific gravity or refractive index, of the reclaimant solution to determine if the buck-up solution should be added and if so, the amount of the buck-up solution to add. The measurement of the one property determines if the reclaimant solution falls within an operating window of a certain range of compositions that may be effective when recycled to the processor. With a two component development solution, measurement of one property of the reclaimant solution that varies as the ratio of the two components changes infers the ratio of the two components in the solution. The amount of buck-up solution that can be added to the reclaimant to bring it to a target composition can be determined from a look-up table based on the composition of the reclaimant and the total volume of the system. But measurement of only one characteristic of the reclaimant solution is not indicative of the proportions of each of the components particularly when the development solution (and reclaimant solution) contains three or more components. This method typically cannot correctly rebalance multi-component solvent solutions to bring all the components within a desired operational composition.
One way to determine if a development solution that is composed of 3 or more components is out of balance is to send samples of the solution for complex analytical testing at a remote location. However, this is time consuming and expensive, and can delay production of printing forms.
In view of the above problems, there arises a need to provide customers that prepare printing forms with an easy, in-house, method that provides immediate feedback to the status of the balance of components in a development solution and/or a reclaimant solution. The method should also provide guidance to the addition/s of buckup solution/s or fresh component solution/s necessary to maintain the proportions of the components in the development solution and/or the reclaimant solution for effective removal of materials from the photosensitive element.